This invention generally relates to a method and apparatus for measuring and processing a characteristic of subsurface earth formations penetrated by a borehole. More specifically this invention relates to a method and apparatus for measuring and processing an acoustic characteristic such as slowness of subsurface sonic waves after these waves traverse earth formations adjoining a borehole or passing through a portion of the subsurface.
Sonic wave information is used by the oil industry to examine and evaluate the earth""s subsurface in the exploration and evaluation of valuable mineral deposits. Sonic waves are generated and recorded in oil well logging. This is called sonic or acoustic logging. The sonic wave measurement taken in well boreholes is typically the formation compressional slowness (the reciprocal of velocity). However, many different acoustic wave types may be measured, for example shear waves or Stonely waves. In the uncased well borehole the compressional head wave or direct wave is the first arrival of the compressional waveforms; the compressional slowness may be derived by measuring the first time of arrival of energy at two acoustic sensors or receivers located a known distance apart. The method does not work well in the presence of noise. In boreholes with a casing or liner, the tube or casing wave also interferes with the detection of the acoustic waves associated with subsurface earth formations.
Acoustic logging is performed in order to resolve the slowness or velocity structure of subsurface earth formations. The subsurface earth formation information accuracy or resolution that is possible is directly related to both the acquisition as well as the processing of the acquired data.
After the sonic data are acquired signal processing methods are needed to estimate the acoustic slownesses of interest associated with earth formations. The acoustic slowness measured between any two receivers is always the average over the distance between them. In acoustic logging, as the receiver separation distance increases, the quality of the slowness measurement increases while the resolution between receivers decreases. This occurs as a result of the averaging of actual slowness variation between the receivers. A short receiver distance aperture provides less averaging while usually giving noisier but potentially higher resolution data.
Another factor that affects the resolution provided by acoustic logging is the wavelength of the acoustic energy measured. It has generally been thought that one cannot resolve variations in the slowness occurring over distances qualitatively much less than a wavelength. An acoustic wavelength, assuming a sound speed of 20,000 ft/s, at 10 kHz is 2 feet. It has been assumed that sonic logging methods may resolve beds on the order of a foot thick or more when the operating frequency is approximately 10 kHz. However, as a practical matter, resolution in conventional acoustic logging has been about 3.5 feet, or approximately the length of the standard receiver array.
In formation evaluation, there is often a need to quantify the acoustic and petrophysical properties of laminated thin beds for better reserve estimation and reservoir characterization of valuable subsurface mineral deposits. Standard array acoustic processing yields a slowness log that tends to smooth, or average, the actual variations over the length of the receiver array (typically 3.5 ft.), obscuring the features that are smaller than the array aperture.
Signal processing techniques have been sought to enhance the vertical resolution of acoustic slowness logs. Hsu and Chang (1987) applied a multiple-shot semblance technique to sub arrays of four receivers and achieved a measurement scale of 1.5 ft. Tang et al. (1994) applied a phase matching method to all waveform pairs across one inter-receiver spacing and achieved a measurement scale of 0.5 ft. Both techniques utilize redundant information in overlapping sub-arrays that span the same depth interval to suppress noise and to improve the vertical resolution.
The drawback in the Hsu and Chang technique is that noise may severely degrade the data output quality using the semblance technique when the number of receivers in the array is less than four. Therefore, it is difficult for this technique to achieve a measurement scale smaller than the aperture of a four-receiver array, typically 1.5 ft. The drawback of the Tang et al. technique is that the phase matching in the frequency domain requires that a high-quality wave phase spectrum be calculated. Acquiring high quality phase spectra may be problematic since examining a long temporal portion of the waveform to calculate the phase spectrum is prone to noise contamination, while examining a short temporal portion to calculate the spectrum may significantly distort the phase spectrum.
Both the Hsu and Chang and the Tang et al. work recognized that the using shorter sub-array apertures to enhance resolution is more prone to noise contamination since there is less move-out and fewer data are used. Therefore, there is less slowness information and less data redundancy. The key in obtaining a reliable, high-resolution acoustic slowness profile using short sub-arrays is to reduce noise contamination by maximizing the redundancy of information in the data.
The present invention is a method for acquiring and processing acoustic waveform data. A waveform matching inversion is performed to obtain formation slowness profiles at various resolutions ranging from the total length of the receiver array to the inter-array receiver spacing. Using overlapping sub-arrays of reduced aperture provides for resolution enhancement. The enhancement is achieved by minimizing the noise contamination effects by maximizing the information redundancy in waveform data. The method achieves this by isolating the wave event of interest and matching the waveform of the event for all possible receiver pairs allowed by the sub-array. The high-resolution slowness curve successfully resolves the laminated features in a geological formation. This invention is a useful tool for evaluating thin beds in laminated formations using borehole acoustic logging.